1. Field of the Invention
Wastewater "biomass" systems normally consist of a mixture of water, organic matter and a variety of bacterial genera whose food, to a large degree, consists of the organic matter component of the biomass and/or other organic waste materials. Anaerobic digestion is widely employed to reduce biomass and/or to treat high (&gt;1000 mg/l) biological oxidation demand (BOD) liquid wastewaters. Progressive destruction of the organic matter in biomass systems also has been made more efficient by introducing various gases that serve as nutrients for bacteria and/or to strip gas molecules such as those of methane, carbon dioxide, ammonia, hydrogen, hydrogen sulfide, etc. from the microbial solutions where such gases occur as waste products during bacterial degradation. Nitrogen, hydrogen and recirculated biomass product gases (e.g., carbon dioxide, methane and hydrogen) have been used for these purposes.
2. Description of Related Art
Anaerobic digestion processes have been utilized to treat and remove organic compounds from waste products such as sewage, sewage sludge, chemical wastes, food processing wastes, agricultural residues, animal wastes and other organic waste materials such as those produced by paper pulping plants. Anaerobic bacteria (or other anaerobic microorganisms) require a virtual absence of oxygen in order to digest organic waste materials. Hence, anaerobic digestion reactors are tightly sealed to prevent entrance of air therein. This airless condition is often referred to in the literature as an "anoxic" condition.
Anaerobic digestion may be carried out in a single reactor chamber or in multiple reactor chambers (e.g., those having two or more physically distinct chambers in which two or more distinct operations are independently carried out). Heat is often added to such reactors in order to maintain adequate temperatures for certain thermophilic or mesophilic bacteria that digest or otherwise breakdown the organic components of a biomass. Mixing of incoming wastes or biomass within such reactors, either by mechanical agitation, or by gas bubbling, also is commonly used to accelerate a wide variety of such anaerobic digestion processes.
The products of anaerobic digestion normally consist of: (1) a gas phase containing carbon dioxide, methane, ammonia, small amounts of certain other gases e.g., hydrogen sulfide and hydrogen (and trace amounts (e.g., less than one tenth of one percent by volume) of other gases such as propane), which, in total, constitute what is commonly referred to as "biogas"; (2) a liquid phase (aqueous in nature) in which ammonia, nutrients and a host of organic gases and inorganic chemicals are dissolved; and (3) a colloidal phase of suspended solids containing undigested organic and inorganic compounds, synthesized biomass and/or bacterial cells.
It also has been long recognized that low anaerobic gas product concentrations within an anaerobic reactor can result in significantly reduced reactor detention times and thus smaller reactor size requirements. Reduction of reactor size, in turn, results in the advantages of reduced capital costs, as well as reduced operating costs. Some workers in this art also have postulated that maintenance of low gas product concentrations serves to increase overall anaerobic treatment efficiencies. They reason that: if the outer surfaces of bacterial cell walls are not covered with gas molecules, this condition will allow newly produced gas molecules to leave such bacteria more quickly--and thereby produce more efficient digestive processes. Moreover, these relatively "gas-bubble-free" bacteria cell walls will permit secondary addition of nutrient molecules into such bacteria (through their cell walls) owing to the fact that the outer cell wall surfaces are not occluded by the presence of gas molecules. These conditions also are thought to cause generation of greater amounts of methane, carbon dioxide, ammonia, hydrogen, etc. gases, and, hence, ultimately produce cleaner liquid effluents that, in turn, can be disposed of more economically. It also should be noted that increased digestion efficiencies result in reduced solids concentrations within the effluent products of such processes. This circumstance serves to reduce the quantity of solid or semi-solid material that must be ultimately transported to a disposal site. Finally, higher treatment efficiencies render the solid and liquid effluents of such processes more amenable to separation of potentially valuable resources (heavy metals and nutrients such as nitrogen-containing ammonia) that may be contained in such effluents.
The degradation of organic matter present in anaerobic systems proceeds by many highly complex chemical processes. Their final stages usually involve processes that result in production of several gases: methane, carbon dioxide, ammonia, hydrogen sulfide and trace amounts of other gases including propane. For example, anaerobic treatment of municipal wastewater ultimately produces a biogas consisting mostly of methane (CH.sub.4) and carbon dioxide (CO.sub.2) along with a sludge product. In these larger anaerobic digestion plants, the biogas byproduct is often burned to provide heat for the digester and/or for local energy production devices. Biogas, however, in and of itself, does not burn particularly well since it has a relatively low BTU value of about 650 BTU/ft.sup.3. Consequently, in order to burn biogas, expensive burner modifications must be made. Moreover, the biogas usually has to be mixed with other, better burning, fuel gases in order to most effectively burn the biogas component of a biogas/fuel gas mixture.
At least three separate stages have been distinguished in anaerobic processes. They can take place in separate chambers of a digester, or they can take place simultaneously in a milieu wherein various microbial genera coexist with organic matter in various stages of degradation. In any case, the first stage of such anaerobic digestion processes takes place when solid complex organic materials such as cellulose, proteins, lignins and lipids are hydrolyzed into simpler, soluble organic molecules such as fatty acids, alcohols, carbon dioxide and ammonia. Such digestion processes also may be aided by extracellular enzymes (i.e., enzymes operating outside of the microbial cells of the biomass) that usually produce pyruvic acid, lactic acid, acetic acid, propionic acid or butyric acid.
The second stage of anaerobic digestion processes, often called "acetogenesis", involves conversion of the fatty acid products of the first stage to acetic acid, propionic acid, hydrogen, carbon dioxide and other low molecular weight organic acids. This stage is carried out by so-called acetogenic bacteria.
The third stage of such digestive processes primarily involves methanization process in which residual metabolites of the preceding two steps are converted into methane by methanogenic bacteria. To this end, at least two distinct bacteria species are employed. One group converts hydrogen and carbon dioxide to methane while a second group converts acetate to methane and bicarbonate (carbon dioxide in solution). Both groups of bacteria are however anaerobic in character. Thus, successful digestion requires a balance between production and consumption of various intermediate materials in each of the three stages of anaerobic digestion.
Those skilled in this art also will appreciate that a "rate-limiting" stage in such processes is that stage in which conversion of such waste materials is the slowest. Both Stage 1 and Stage 3 can be rate - limiting in anaerobic digestion processes. For example, in the first stage of certain anaerobic digestion processes, the breakdown of certain complex organics such as cellulose (e.g., those from paper pulping operations) to organic fatty acids is the most inherently slow step--even though the microorganisms that provide the enzymes to catalyze this particular breakdown grow relatively quickly. This follows from the fact that a lignin component in pulp waste products prevents access of such enzymes to their cellulose component and thereby slowing digestion of these complex organics. Other complex organics simply do not biodegrade easily; consequently, long retention times and low loading rates are needed for their conversion in this step. Consequently, the presence of such complex organics usually implies that Stage 1 will rate limit certain anaerobic digestion processes.
By way of comparison, conversion of organic fatty acids to organic volatile acids (primarily acetic acid) in Stage 2 is generally much easier and, consequently, Stage 2 is rarely the rate limiting step of an anaerobic process. In other words, the organisms needed for Stage 2 usually grow fast, and quickly break down the fatty acid intermediates in the biomass. On the other hand, the third stage is usually much slower than the second. This follows from the fact that methane-forming bacteria normally grow relatively slowly and are much more sensitive to various environmental factors compared to the microorganisms that carry out Stages 1 and 2. Indeed, reactions of methane-forming bacteria are usually "the rate-limiting reaction" in most anaerobic reaction systems. Hence, the third stage is usually considered the overall rate-limiting stage of most anaerobic digestion processes. Consequently, the goal of most industrial and/or environmental protection anaerobic digestion processes is to maintain, at the highest level possible, without sacrificing overall process stability, a rate of "food" consumption and production for the microorganisms of the rate-limiting, third stage.
Next, it should be noted that many anaerobic biomass systems also contain ammonia and/or hydrogen sulfide gases. Their presence is known to inhibit the purifying operations of many organic material-containing wastewater/biomass systems. For example, it is known that during the second stage of many anaerobic digestion processes, hydrogen sulfide-reducing bacteria frequently residing in such biomass systems may consume a large proportion of the volatile fatty acids--to the detriment of this second, or acetogenic, stage, and, hence, to the detriment of the desired production of acetic acid. This results in higher proportions of volatile fatty acids escaping the degradation process. Sulfate-reducing bacteria also are known to be harmful to the third stage: firstly, because they consume hydrogen and carbon dioxide to the disadvantage of the methanogens, and secondly, because the hydrogen sulfide produced by such sulfate-reducing bacteria inhibits the function of various acetoclastic bacteria in many such systems. Analogous, but usually less severe, inhibitions are known to occur as a result of the presence of various other gases (e.g., methane, ammonia, carbon dioxide, etc.) produced by anaerobic digestion of various organic material-containing wastewaters. Thus, all of these facts and circumstances indicate that the inhibition phenomena associated with the presence of various gas products of anaerobic digestion processes (e.g., methane, ammonia, carbon dioxide, hydrogen sulfide, etc.) can be greatly diminished by rapidly removing these gases from the digestive process--and most preferably from the most sensitive stage thereof, the third, or methane production, stage.
Many academic, industrial and patent references have recognized one or more of the above-noted gas inhibition problems and have suggested various solutions to them. For example, within the academic and industrial literature, the following articles are particularly instructive: FinLey, C. D., and R. S. Evans, Anaerobic Digestion--the Rate Limiting Process and the Nature of Inhibition, Science, 1975, Vol. 190, p. 1088; McCarty, P. L., Anaerobic Waste Treatment Fundamentals, Part I, Public Works, 1964, p. 107 and Obayashi, A. W., and J. M. Gorgan, Management of Industrial Pollutants by Anaerobic Processes in Industrial Waste Management Series, W. James (ed.), Lewis Publishers, Inc., Chelsea, Mich., 1985.
FinLey and Evans reported that a "fourth" rate controlling step may be present in anaerobic digestion--namely a step based upon the fact that dissolved gas products (e.g., CH.sub.4 +CO.sub.2) must undergo a transfer from the liquid to the gas phase. That is to say that, even though most investigators consider the third step (i.e. biological conversion of organic acids, such as acetic and propionic acid into methane and carbon dioxide) to be the rate controlling reactions, Finney and Evans proposed that a fourth step i.e., transfer of dissolved gases from the liquid phase to a gas phase or, alternately, the rate of removal of these gas bubbles away from bacterial cell walls, is actually the rate limiting step in anaerobic digestion.
To prove their hypothesis, Finhey and Evans conducted controlled anaerobic digestions under the presence of both increased agitation and, more importantly, under vacuum conditions. The presence of a vacuum resulted in a lowering of the partial pressure of CH.sub.4 and CO.sub.2 above the liquid. By lowering the partial pressure above the liquid, the concentration of dissolved CH.sub.4 and CO.sub.2 in the liquid phase was also lowered. Thus the bacterial degradation products of CH.sub.4 and CO.sub.2 clinging to microbial cell walls were more rapidly transferred away from those cell walls into a aqueous phase and ultimately out of the system. When they compared their reaction rates to established and well documented reaction rates by other workers, Finney and Evans observed that their vigorous agitation and lower pressure conditions resulted in a 600% increase in the reaction rate. They attributed this increased reaction rate to the faster removal of CH.sub.4 and CO.sub.2 bubbles away from the bacterial cell walls thereby effectively increasing the "bacterial surface area available for permeation processes." In other words the bacteria had more surface area available to allow intake of their food sources of acetic and propionic acids.
Those skilled in this art, however, also will appreciate that application of a strong vacuum (e.g., 100 Torr) such as that employed by Finney and Evans, in a commercially sized digester, may create operating problems due to vessel collapse and/or foaming conditions that tend to be created by such vacuum conditions. For example, this problem was noted by Keenan, C. W. and Wood, J. H., 1967, General College Chemistry, 3rd Ed., Harper & Row, NY, 814 p. (242).
It also should be noted in passing that the Finhey and Evans gas stripping concept and experimental evidence is supported by the chemical principle established by LeChatelier in 1885 (LeChatelier's Principle) which states that "when a stress is brought to bear on a chemical system at equilibrium, the system tends to change so as to relieve the stress." Therefore, for the reaction of EQU A+B.fwdarw.C+D
if C and/or D are removed from the system then more A+B are converted to C+D.
As applied to anaerobic digestion, the following system may be conceptualized: ##STR1##
Thus if CH.sub.4 and CO.sub.2 are removed from an anaerobic system (i.e., "stress" is applied to the system), additional acetic acid will be consumed by the bacteria in order to re-establish equilibrium conditions. Another way of looking at LeChatelier's principle in the context of anaerobic digestion, is that no living system, no matter how simple, enjoys being forced to reside in its own waste products.
Application of LeChatelier's principle was therefore well demonstrated by the experimental work of Finney and Evans (1975), including their application of a vacuum that resulted in more rapid removal of CH.sub.4 and CO.sub.2 from their experimental system and thereby allowing the methogenic bacteria to consume more acetic (and propionic) acid, and thereby increasing their overall reaction rates. These authors recognized that microbial growth could be inhibited by absorption of toxic compounds onto the surface of cells and thereby affecting cell wall permeability to gases and/or dissolved feed passing either into, or out of, such cells. They also noted that as CH.sub.4 and CO.sub.2 are produced by bacteria and migrate through a cell wall on a molecular basis, they tend to remain attached to the cell wall pending their "escape" into the aqueous solution. In any case, Finney and Evans proposed that by remaining attached to the cell wall these product gases may effectively decrease the surface area of the bacteria, inhibit cell permeability and reduce a cell's capability of absorbing needed nutrients.
The academic literature also has long recognized that some trace amounts of propane are produced by anaerobic digester systems. For example, a leading text in this general area: Methane Production From Waste Organic Matter, by Stafford et al., CRC Press, Inc., Boca Raton, Fla. (1980) ("the Stafford reference") states (on page 114) that some "small quantities" of propane may be formed by "polymerization" of methane in an anaerobic digester. It also might be noted that Stafford et al. also state that anaerobic digesters usually make 60-70% methane and 30-40% CO.sub.2 along with "small amounts" of H.sub.2 S and H.sub.2.
The patent literature also has long recognized the fact that certain gases can be used to "strip" various gases (e.g., methane, carbon dioxide and hydrogen sulfide) from anaerobic biomass systems. The patent literature also has well recognized use of certain gases as nutrients for various microorganisms residing in anaerobic digestion systems. For example, U.S. Pat. No. 4,289,625 (the '625 patent) teaches use of a "hybrid, bio-thermal" system comprised of an anaerobic digester unit and a thermal gasifier unit. This patent teaches that various gases produced by heating a solids component (sludge) of a biomass system in a thermal gasifier unit can be re-introduced into the system's anaerobic digester unit. This system seeks to "provide a process for high conversion of carbonaceous material in biological feed stocks to gas products" (column 2, lines 39 and 40). It also should be noted that this reference teaches that the thermal gasifier unit produces 94 mole percent methane (column 8, line 58). In effect, the anaerobic digester system of the '625 patent achieves greater methane production per unit of feed by "digesting" and "cracking" the anaerobic sludge material and, secondarily, by feeding the thermal gasifier's, gaseous products (H.sub.2, CO), back to the digester unit as food sources for the microorganisms residing therein. Some of these gaseous products are characterized as "C.sub.x H.sub.y " in the '625 patent, but no particular emphasis is laid upon a propane gas component that may fall within the highly generalized term "C.sub.x H.sub.y "--especially since a thermal gasifier will produce an extremely varied source of hydrocarbons due to its "cracking" ability.
The '625 patent acknowledges that the act of recycling product gases that "principally contain methane and carbon dioxide", "may inhibit methane production due to mass action" (column 1, lines 52-58) i.e., the '625 patent's recognition of Le Chattier's principal in its processes. It might also be noted that no mention is made in the '625 patent of any deleterious affects to the rate of methane production due to "mass action". Nor does the '625 patent teach or suggest introduction of a soluble gas (such as propane) which is not a food source for the bacteria; nor does the '625 patent teach introduction of a soluble gas into its digester unit to achieve a beneficial effect on the rate of methane production therein. The importance of the distinction between the "rate" of biogas production versus the total "amount" of biogas production will be more fully developed in the later descriptions of applicant's processes. For the moment, however, suffice it to say that applicant's processes seek to increase the rate of biogas production relative to the rates of biogas production achieved by processes such as those described in the '625 patent. It also should be noted at this point that, by way of contrast with the '625 patent, applicant's process does not seek to provide "higher methane production per unit of feed", but rather accelerated rates of digestion.
With respect to the subject of "stripping" gases from anaerobic systems, many patent references teach processes for stripping free hydrogen sulfide content of an anaerobic digester. This removal of hydrogen sulfide may take place either by starting with a germination gas or by starting with a treated effluent leaving a digestion reactor. The patent literature also has recognized that, after removal of hydrogen sulfide, the purified effluent may be partially recycled back to the reactor. In such processes, acid formation, sulfate reduction and methanogenesis usually all take place, simultaneously, within a single reactor unit.
U.S. Pat. No. 5,037,551 teaches bubbling oxygen-enriched gas through a first digestion zone and a low-molecular-weight alkane through a second zone of an anaerobic digester.
U.S. Pat. No. 4,198,292 teaches increased yield of anaerobic digestion by placing a slight vacuum over the biomass system.
U.S. Pat. No. 4,655,924 teaches use of microorganism carrier materials in digester systems.
European Patent EP-0,241,999, describes an anaerobic fermentation based upon successively passing an effluent through two fixed-culture reactors. Acetification takes place in the first reactor. This is coupled with biological reduction of any sulfates present. The effluent leaving the first reactor is then freed of hydrogen sulfide gas by stripping it by means of an inert gas in an intermediate structure before the effluent enters a second reactor. The pH of this system is maintained between 6.5 and 6.7 to promote both sulfate reduction and hydrogen sulfate stripping.
U.S. Pat. No. 5,298,163 teaches that a "neutral" gas (no disclosures are made as to the exact identity of such "neutral" gas) can be introduced in a biodegration process in order to strip or otherwise displace hydrogen sulfide gas from a biomass system.
U.S. Pat. No. 5,015,384 teaches an anaerobic digestion process that employs an injection of "anoxic" gases to strip carbon dioxide gas from the system so that its pH remains neutral or nearly so. This reference states: "The primary requirement is that the gas be anoxic, i.e., not contain oxygen or other constituents toxic to the anaerobic bacteria." This reference does not, however, specifically identify or exemplify the anoxic gases--but it does suggest that carbon dioxide gas bubbles attached to the outer surface of bacteria cell walls may be removed by the overall stripping process described therein.
U.S. Pat. No. 4,826,600 teaches a process for altering the pH of a anaerobic system (e.g., one degrading sewage) by using methane gas to strip carbon dioxide gas from the system. This process may also employ an "inert" gas to aid in the withdrawal of gaseous products. The preferred "inert" gas is methane.
U.S. Pat. No. 3,383,309 teaches a process for anaerobic digestion of sewage sludge in a digester wherein biodegradable solids in a sludge are converted (by a first group of bacteria) into fatty acids. The resulting fatty acids are thereafter transformed into methane gas and gaseous carbon dioxide (by methane and carbon dioxide forming bacteria). The novelty of this process resides in the step of removing the digester gases from the digester, adding energy to at least a portion of these digester gases, cracking them into hydrogen and other gases such as methane, and then introducing at least a portion of the hydrogen gas into the sludge in the digester for assimilation by the methane forming bacteria, with a resultant increase in fatty acid transformation by the bacteria.
U.S. Pat. No. 5,116,506 discloses a method of treating liquid waste. The method includes the steps of (1) providing a reactor having a gas permeable membrane that divides the reactor into a liquid compartment for the liquid waste and a gas compartment for a gas component; (2) providing a biofilm layer on the liquid compartment side of the membrane (the layer comprising a first layer of aerobic organisms adjacent the membrane, and a second layer of anaerobic organisms between the aerobic layer and liquid); (3) introducing an oxygen containing gas into the gas compartment and allowing it to diffuse through the membrane; and (4) introducing a liquid waste into the liquid compartment and allowing it to react with the biofilm layer.
U.S. Pat. No. 5,310,485 teaches an anaerobic wastewater treatment process wherein biogas--generated in a digester unit, and in a flotation container--is recirculated through a gas entrainment system that is positioned away from the flotation container.
U.S. Pat. No. 5,185,079 teaches an anaerobic reactor that removes biogas from the top of the reactor and re-introduces it into the bottom of said reactor. This reference also notes the beneficial effects of applying a vacuum during its settling phase to promote removal of gas bubbles attached to bacterial cell walls.
U.S. Pat. No. 4,780,415 discloses an anaerobic degradation process having certain novel biogas collection and reintroduction steps.
U.S. Pat. No. 4,511,370 teaches a process for utilization of household garbage. It includes the steps of collecting carbon dioxide and methane gas produced by the process, separating them from each other and then re-introducing the methane component of the gas separation back into the process.
U.S. Pat. No. 4,311,593 teaches a process for treatment of wastewater wherein, as the wastewater flows downwardly through a digester unit, methane is released and bubbles to the surface where it is either collected for combustion in an energy production process, or is recirculated. A pressure of about one pound per square inch is kept on the digester in order to keep carbon dioxide dissolved in the system's liquid effluent.
U.S. Pat. No. 4,372,856 discloses a process for anaerobic digestion of waste wherein sludge is sparged with methane gas in order to stimulate the growth of anaerobic bacteria and, thus, greater production of biogas. This reference also teaches that biogas may be stripped of its "undesired" carbon dioxide and hydrogen sulfide components by passing it through a scrubbing liquid prepared by use of ammonia produced by another phase of the overall process.
U.S. Pat. No. 4,482,458 teaches a process for anaerobic treatment of wastewater that includes the steps of collecting a biogas product of the anaerobic treatment, compressing it and then re-introducing the compressed biogas back into the treatment system.
U.S. Pat. No. 3,242,071 teaches a stirring method that is particularly useful in operating a digester.
U.S. Pat. No. 4,897,195 teaches use of rotary digestion modules to produce carbon dioxide, methane and nitrogen based fertilizers by anaerobic digestion of various waste materials.
U.S. Pat. No. 4,983,297 teaches use of an anaerobic treatment for water separated from crude oil.
U.S. Pat. No. 5,232,596 teaches a biodegradation process wherein off-gas components of said process are recirculated back to the bioreactor.
The use of propane in anaerobic digesters is also noted in the trade literature. For example, the January 1996 issue of the trade magazine Butane-Propane News (p. 32) reports that propane can be bubbled through anaerobic digesters to accelerate their reaction rates. The operating parameters of such systems, however, are not disclosed therein.
These academic, trade and patent references suggest that much work has been done to improve the performance of anaerobic digesters--and, indeed, much has been accomplished. However, further improvements in this art are still being sought on many fronts--and are always welcome when, in fact, they are achieved. Applicant's present contribution to this art resides in his finding that when propane gas is injected into anaerobic digestion systems in certain concentrations hereinafter more fully described, anaerobic digestion performance is improved far beyond those levels of improvement achieved by introduction of methane, carbon dioxide or hydrogen stripper gases. The herein described processes also are based upon applicant's recognition that even though many references teach use of certain gases (e.g., methane, carbon dioxide and hydrogen) to strip carbon dioxide and hydrogen sulfide gases from anaerobic systems, the full beneficial effects of removing such gases from anaerobic systems has not been heretofore fully achieved. This failure follows from the fact that large concentrations of certain stripper gases used in many prior art processes produce deleterious, inhibitory effects on the biomass microorganisms due to the principles of operation of Le Chatelier Principle in such anaerobic systems for the reasons previously noted. In effect, applicant's process seeks to circumvent the problems associated with the inhibitory effects of increased methane, carbon dioxide and hydrogen concentrations in anaerobic microorganisms--while still providing a means for stripping gaseous products from the outer cell walls of such microorganisms. Applicant's processes also serve to force gases that are dissolved in the aqueous phase of the biomass out of the overall anaerobic digestion system.